It is not uncommon for a user's premises to be connected to a public telephone system via a pair of twisted pairs (i.e. 4 wires in total). Usually one such pair is redundant as only a single pair is needed to provide Plain Old Telephony Services (POTS) or DSL services. However, there have been many proposals for how best to utilise the redundant pair in such a case. These proposals range from the very straightforward approach of using each pair as a separate DSL connection and “bonding” the connections (bonding is discussed in slightly more detail below) through to more exotic and speculative approaches such as that described in EP 1 733 550 in which just one of the 4 wires is used as a reference wire and 3 channels are formed each carrying a different signal relative to the single common reference wire which are then “vectored” and bonded. Bonding involves combining the separate underlying channels at a layer above the physical layer (e.g. at the transport or network layer (OSI model) or (approximately equivalently) at the TCP/IP layer) to present to the application layer a single connection having a data rate approximately equal to the sum of the individual data rates of the underlying connections. Vectoring is a well-known DSL technique which is briefly discussed below after firstly discussing DSL generally.
DSL technology takes advantage of the fact that although a legacy twisted metallic pair (which was originally installed to provide merely a Plain Old Telephone Services (POTS) telephony connection) might have been intended to carry signals at frequencies of only up to a few Kilohertz, in fact such a line can often reliably carry signals at much greater frequencies. Moreover, the shorter the line, the greater is the range of frequencies over which signals can be reliably transmitted (especially with the use of technologies such as Discrete Multi-Tone (DMT), etc.). Thus as access networks have evolved, telecommunications network providers have expanded their fibre optic infrastructure outwards towards the edges of the access network, making the lengths of the final portion of each connection to an end user subscriber (which is still typically provided by a metallic twisted pair) shorter and shorter, and thus giving rise to correspondingly greater and greater bandwidth potential over the increasingly short twisted metallic pair connections—without having to bear the expense of installing new optic fibre connections to each subscriber.
However, a problem with using high frequency signals is that a phenomenon known as crosstalk can cause significant interference, reducing the effectiveness of lines to carry high bandwidth signals in situations where there is more than one metallic pair carrying similar, high-frequency, signals in close proximity to one another. In simple terms, the signals from one wire can “leak” onto a nearby line carrying similar signals and appear as noise to the other line. Although crosstalk is a known problem even at relatively low frequencies, the magnitude of this effect tends to increase with frequency to the extent that at frequencies in excess of a few tens of Megahertz (depending on the length of the lines in question), the indirect coupling (e.g. from a near end of a second line to a remote end of a first line) can be as great as the direct coupling (e.g. from the near end of the first line to the remote end of the first line).
In order to alleviate the problems caused by crosstalk (especially Far End CrossTalk or “FEXT” as it is known) a technology known as vectoring has been developed in which knowledge of the signals sent over crosstalking lines is used to reduce the effects of the crosstalk. In a typical situation a single DSLAM acts as a co-generator of multiple downstream signals over multiple crosstalking lines and also as a co-receiver of multiple upstream signals from the same multiple cross-talking lines, with each of the lines terminating at a single Customer Premises Equipment (CPE) modem such that no common processing is possible at the CPE ends of the lines. In such a case, downstream signals are pre-distorted to compensate for the expected effects of the crosstalking signals being sent over the neighbouring crosstalking lines such that at reception at the CPE devices the received signals are similar to what would have been received had no crosstalking signals been transmitted on the crosstalking lines. Upstream signals on the other hand are post-distorted (or detected in a manner equivalent to their having been post-distorted) after being received at the co-receiver (the DSLAM) in order to account for the effects of the cross-talk which has leaked into the signals during their transmission.
It has been known in theory for a long time (see for example “DSL Advances” by Starr, Sorbara, Cioffi and Silverman published by Prentice Hall 2003—p. 344 and reference to “full vectoring”) that improved cross-talk cancellation can be achieved if it is possible to coordinate the signals at both ends of the coordinated channels (see also p. 373 and FIGS. 11.38 and 11.39 of the same reference where it explains that “when either end of the link can be coordinated at that same end, then NEXT [Near End Cross Talk] can be cancelled with a simple multidimensional echo canceler” (thus removing the need to transmit upstream and downstream signals at different frequencies and thus potentially increasing the overall data rates considerably—ignoring the impact of regulations specifying Power Spectral Density (PSD) masks for telephony products located in an access network)).
Such vectoring techniques can deal very successfully with situations where the indirect coupling is significantly weaker than the direct coupling. However, as the relative strengths of the direct and indirect coupling approach each other, vectoring is less able to function effectively.
WO2013026479 applied for by Ericsson proposes a method of transmitting signals in such a situation (i.e. where an indirect coupling is comparable to a direct coupling for a given line) which involves transmitting signals intended for reception by a single CPE device (a first CPE device) onto both the line directly coupled to the first CPE device and onto a crosstalking line coupled only indirectly to the first CPE device (it being directly coupled to a second CPE device). A Time Division Multiplexing (TDM) method is used to enable data to be sent (in different time slots) to the two respective CPE devices (with data being sent over both wires at the same time to only one of the CPE devices at a time). In order to ensure that the two signals constructively interfere at the receiving CPE device, the same signal as sent over one line is pre-distorted (e.g. to introduce a delay and/or phase change) before being sent over the other to account for changes in the direct vs the indirect coupled paths.
EP2091196 by Alcatel-Lucent provides a method to inject signals into the phantom mode formed between two Twisted Metallic Pairs (TMPs). The injected signals are the same as those sent onto one of the TMPs, but phase-rotated so that when converted and coupled into the differential mode, they interfere constructively with the signals sent directly over the respective one of the TMPs in the normal differential mode.
WO 2005/004429 by Globespan Virata Inc describes a method for transmitting data over at least two bonded channels in a flexible manner which provides multiplexing when possible and exploits diversity when needed to improve resilience. In particular the method may comprise transmitting a first symbol stream over a first tone in a first bonded channel and over a second tone in a second bonded channel, wherein the first tone is bonded to the second tone, and the transmissions over the first tone and the second tone are substantially parallel; transmitting a second symbol stream over a third tone in the first bonded channel; and transmitting a third symbol stream over a fourth tone in the second bonded channel, wherein the third tone is bonded to the fourth tone, and the transmissions of the second symbol stream and the third symbol stream are substantially in parallel.